Atomic layer deposition (ALD), also known as sequential pulsed chemical vapor deposition (SP-CVD), atomic layer epitaxy (ALE) and pulsed nucleation layer (PNL) deposition, has gained acceptance as a technique for depositing thin and continuous layers of metals and dielectrics with high conformality. In an ALD process, a substrate is alternately dosed with a precursor and one or more reactant gases so that reactions are limited to the surface of a substrate. Uniform adsorption of precursors on the wafer surface during the ALD process produces highly conformal layers at both microscopic feature length scales and macroscopic substrate length scales, and achieves a high density of nucleation sites. These attributes result in the deposition of spatially uniform, conformal, dense and continuous thin films.
Although ALD techniques support deposition of conformal thin layers, a drawback of the technique is the low average deposition rate, which is related to the need to repeat several cycles having finite durations. For example, the repeated cycle of precursor and reactant adsorption and the intervening chamber purges is time consuming, which results in reduced throughput relative to conventional deposition techniques. Specifically, an ALD sequence includes at least two purge pulses and these purge pulses are typically the most time consuming portion of the ALD sequence. Therefore, improvements in ALD equipment have focused on techniques to decrease the time needed to complete a purge pulse.
The most logical solution to decreasing the duration of the purge pulse is to flow the purge gas at higher speeds through the reactor, which may be achieved by increasing the flow rate of the purge gas. Typical flow rates used in the industry are several standard liters per minute (SLM) (e.g., approximately 2.5 SLM) at pressures of between approximately 0.2 and approximately 20 Torr. These flow rates lead to substantially higher gas flow speeds than obtained in conventional CVD processes.
One of the effects of increasing purge gas flow speed is the occurrence of turbulence in the gas injector. Typically, the turbulence occurs in an expansion zone of a gas injector near an inlet used to supply the purge gas. Turbulence in the expansion zone may cause the flow pattern of the purge gas across a conventional diffuser plate to be altered. Specifically, the fraction of the total flow passing through the openings in the diffuser plate located near the turbulent zone decreases significantly. The decrease in gas flow through openings near the turbulent zone when compared to the gas flow through openings located away from the turbulent zone may create an uneven distribution of precursor during a doping, which ultimately forms a non-uniform film on a substrate. Additionally, recirculation of gas in the expansion zone caused by the turbulence leads to inefficient purging of the precursors from the expansion zone, which may cause gas phase reactions that form a powder in the expansion zone.